1. Field of the Invention
The present invention relates to an optical transmission apparatus for a large-capacity and long-distance optical transmission system.
2. Description of the Related Art
Along the development of a multimedia network, demand for communication traffic is increasing dramatically, and a transmission system that carries out multiple relay and amplification of an optical signal using an optical amplifier plays an important role of economizing a communication system in a multimedia society.
Recently, a wavelength division multiplexing (WDM) system is actively introduced in a metro core network in which cost and size are important factors. A synchronous optical network add drop multiplexer (SONET ADM) that is used as a conventional optical transmission apparatus in a ring network is now being replaced with an optical add drop multiplexer (OADM) system using an OADM device having a protection function.
FIG. 10 is a configuration diagram of a network of the OADM system. An OADM system 1000 consisting of a ring network has plural (m) nodes n (n1 to nm) on a transmission path 1001, and each node has an OADM 1002. The OADM 1002 drops an optical signal having an optional wavelength to the transmission path 1001, and adds an optical signal having an optional wavelength from the transmission path 1001 to the OADM 1002. Usually, the OADM 1002 disposed in each node n (n1 to nm) has optical amplifiers that compensate for an insertion loss of an optical signal of the OADM at a pre-stage and a post-stage of the OADM 1002. Specifically, a pre-amplifying unit 1003 is provided at the pre-stage, and a post-amplifying unit 1004 is provided at the post-stage of the OADM 1002.
At the time of initially introducing the OADM system 1000, the introduction cost is required to be minimized. Thereafter, when the demand for metro communication traffic increases, a function of the OADM system 1000 is required to be expanded (upgraded). It is required that a part of the provided function can be used at the initial operation period of the system, and that thereafter the function can be sequentially expanded to meet requirements for a long communication distance and a large capacity.
To smoothly upgrade the function, an optical circuit configuration that can be upgraded at least without changing the optical circuit of a main signal system is required. The optical circuit should have a configuration in which optical parts can be increased at the subsequent upgrading time for reducing the initial introduction cost. For example, there is a method in which at the initial introduction time, only a rare-earth-doped optical fiber amplifier is used as an optical amplifier by anticipating the upgrading, and in which optical signals in a transmission path and a dispersion compensation fiber are Raman amplified at the subsequent upgrading time.
The upgrading is specifically carried out based on requirements for increasing a ring size (increase in the number of nodes, and increase in the length of a transmission path), and increasing capacity (upgrading of a bit rate, and expansion of a signal band). In carrying out the upgrading to meet the requirements for a large capacity and a long distance, noise characteristic expressed by an optical signal to noise ratio (OSNR) in each node n (n1 to nm) needs to be improved based on elements such as the increase in a bit rate and the increase in a ring size.
For example, in the transmission system including the OADM 1002 disposed in each of the nodes n1 to nm of the OADM system 1000, a technique of improving the OSNR in each node n (n1 to nm) is also used to meet the requirement for upgrading (for example, see Japanese Patent Publication No. 3589974). To increase the transmission distance, it is effective to provide an optical amplifier that can increase the OSNR. The OSNR is defined by an input optical power level of a medium that generates noise light and a noise figure NF (OSNR=input optical power level (−) noise figure NF (−) 10 log(h·ν·Δf)). In other words, to improve the OSNR, it is necessary to increase the input optical power level of the medium that generates noise light or reduce the noise figure NG of the noise medium.
FIG. 11 is a configuration diagram of a conventional optical transmission apparatus in an OADM system. A configuration of one node n (n1 to nm) is shown in FIG. 11. In a node 1100, the transmission path 1001, the pre-amplifying unit 1003, the OADM 1002, and the post-amplifying unit 1004 are disposed in this order from the upstream (left side). The OADM 1002 optionally adds or drops an optical signal that is wavelength-multiplexed on the transmission path 1001. The pre-amplifying unit 1003 and the post-amplifying unit 1004 are provided at the input side and the output side respectively of the OADM 1002 to compensate for an insertion loss of the optical signal by the OADM 1002.
A dispersion compensation fiber 1111 provided in the pre-amplifying unit 1003 compensates for degradation in the transmission characteristic of a wavelength-multiplexed optical signal generated due to a difference in the transmission speed of the wavelength-multiplexed optical signal for each wavelength. In the OADM system 1000 (see FIG. 10), in general, each node 1100 has the dispersion compensation fiber 1111 from the viewpoint of dispersion management.
The pre-amplifying unit 1003 compensates for a large amount of dispersion when the transmission path 1001 has a long length (for example, 80 kilometers). The insertion loss of an optical signal in the dispersion compensation fiber 1111 also increases (for example, 10 decibels) accordingly. To avoid degradation in the OSNR due to the insertion loss when the dispersion compensation fiber 1111 is disposed, the pre-amplifying unit 1003 has two optical amplifiers 1112a and 1112b, and has the dispersion compensation fiber 1111 between these optical amplifiers 1112a and 1112b. An erbium-doped fiber amplifier (EDFA) is used for the optical amplifiers 1112a and 1112b. Photodetectors 1113c and 1113d of each optical amplifier detect received optical power via add drop multiplexing units 1113a and 1113b. A controller 1113e controls gains or outputs of the optical amplifiers 1112a and 1112b based on the detected received optical power.
Since the OADM 1002 controls the optical output at a constant level, a simple optical amplifier having a fixed gain, that is, a simple optical amplifier 1121 excluding the function of a variable optical attenuator (VOA), is used in the post-amplifying unit 1004. This optical amplifier 1121 also consists of an EDFA, and includes the add drop multiplexing units 1113a and 1113b, the photodetectors 1113c and 1113d, and the controller 1113e, like the optical amplifiers 1112a and 1112b. The controller 1113e controls a gain or an output of the optical amplifier 1121 based on the detected received optical power.
It is known that Raman amplification of an optical signal in the transmission path 1001 and the dispersion compensation fiber 1111 is effectively carried out to upgrade for larger capacity and the longer distance. Therefore, Raman amplification pump light multiplexers 1114a and 1114b are prepared in advance on the transmission path of the main signal from the initial stage of the introduction of the OADM system 1000. At the upgrading time, Raman amplification pump light sources 1115a and 1115b are provided additionally, thereby executing Raman amplification. A WDM coupler is used for the pump light multiplexers 1114a and 1114b. 
A variable optical attenuator (VOA) 1116 is provided at the input side of the pre-amplifying unit 1003. The variable optical attenuator 1116 is provided to automatically compensate for an optical power component extracted from an input dynamic range in the pre-amplifying unit 1003 when the transmission path 1001 has a short length. Since the transmission path 1001 has various lengths to meet the requirement of system users, it is necessary to broadly compensate for a loss of transmission length.
However, the above OADM system 1000 cannot meet the requirement for a large improvement in the OSNR at the time of expanding the function (upgrading).
The requirement for the improvement in the OSNR cannot be met because noise light of the optical amplifiers is accumulated along the increase in the number of the nodes 1100, thereby degrading the OSNR. When the length of the transmission path 1001 increases, transmission path loss increases, thereby degrading the OSNR. When the capacity is increased by upgrading the bit rate, the increased bit rate expands the optical signal spectrum, and the noise component increases, thereby degrading the OSNR. When the capacity is increased by expanding the signal band, the Raman amplification of an optical signal in the transmission path 1001 and the increased compensation of gain deviation degrade the OSNR.
According to the conventional configuration of the node 1100 shown in FIG. 11, the dispersion compensator (the dispersion compensation fiber) 1111 is disposed in the pre-amplifying unit 1003. According to this layout configuration, at the initial introduction time, due to the insertion loss of an optical signal in the dispersion compensation fiber 1111, to avoid degradation in the OSNR in the pre-amplifying unit 1003, the pre-amplifying unit 1003 requires the two stages of the optical amplifiers 1112a and 1112b. This complicates the configuration of the optical circuit.
At the upgrading time, the OSNR increases in only the pre-amplifying unit 1003, and this is not effective to improve the OSNR. Specifically, the Raman amplification of an optical signal in the transmission path 1001 and the dispersion compensation fiber 1111 in the above configuration only increases the input level in the pre-amplifying unit 1003. Therefore, only the OSNR in the pre-amplifying unit 1003 can be improved. To effectively improve the OSNR, it is necessary to improve the OSNR in both the pre-amplifying unit 1003 and the post-amplifying unit 1004. Consequently, the OSNR cannot be improved effectively in the above configuration.
Regarding the number of optical amplifiers, it is necessary to meet the required system gain width with a smaller number of optical amplifiers. The length of transmission paths of users ranges from long to short. Therefore, there is a range in system gains that are to be supported by optical amplifiers. Optical amplifiers that can be used for wavelength division multiplexed lights are expensive. The configuration in which plural optical amplifiers 1112a, 1112b, and 1121 are provided as shown in FIG. 11 increases cost. When the transmission path 1001 has a short length, the variable optical attenuator 1116 is provided at the input side of the pre-amplifying unit 1003 to reduce the number of menus of the optical amplifiers. With this arrangement, the optical power level exceeding the input dynamic range in the pre-amplifying unit is automatically compensated for. The number of menus means the number of optical amplifiers having different characteristics corresponding to the input power and the like. Optical amplifiers are selected from among those having different characteristics. In this case, the OSNR in the pre-amplifying unit 1003 is degraded due to the insertion loss of an optical signal attributable to the disposition of the variable optical attenuator 1116.
There is an example in which the dispersion compensation fiber 1111 is used for a variable dispersion compensator. However, an optical circuit configuration that effectively increases the OSNR is not provided. In the configuration according to the conventional technique, loss variation in both the transmission path 1001 and the dispersion compensation fiber 1111 is absorbed based on only the configuration at the pre-amplifying unit 1003 side. At the upgrading time, distribution Raman amplification is carried out to the transmission path 1001, and concentrated Raman amplification of an optical signal is carried out in the dispersion compensation fiber 1111, thereby increasing the OSNR in only the pre-amplifying unit 1003.
In the Raman amplification of an optical signal in the dispersion compensation fiber 1111 provided in the pre-amplifying unit 1003, the Raman amplification is carried out at a high level of optical power input to the dispersion compensation fiber 1111. Therefore, a waveform (an eye pattern) of an optical signal collapses, and nonlinearity degrades the transmission characteristics. Further, due to the insertion loss (about a few decibels, in general) attributable to the provision of the variable optical attenuator 1116, the OSNR in the pre-amplifying unit 1003 is degraded.
Within the node 1100, the pre-amplifying unit 1003 operates at a high rate to recover the loss and to improve the OSNR at the pre-amplifying unit 1003 side. Therefore, the optical circuit has a complex configuration in the pre-amplifying unit 1003. Since the OSNR is determined obviously in both the post-amplifying unit 1004 and the pre-amplifying unit 1003, it is difficult to substantially improve the OSNR according to the method of increasing the OSNR only in the pre-amplifying unit 1003.